The benzpyrene (BP)-deoxyribonucleoside products were isolated from the DNA of hamster embryo (HE) cells and human fibroblasts (WI-38) treated with H3-BP. The products were similar to those previously isolated from mouse embryo cultures. In HE cultures pretreated with 7,8-benzoflavone, an inhibitor of aryl hydrocarbon hydroxylase (AHH), the amount of BP-DNA products formed decreased in proportion to the decrease in BP metabolism and there were no qualitative changes in the BP metabolites or BP-deoxyribonucleoside products formed. Treatment of HE cells with inducers of AHH did not enhance metabolism or binding of BP, nor alter the types of products formed. WI-38 cells metabolized low concentrations (O.5 m micron moles/ml) of BP completely, but slowly. After 6 days incubation, 70% of the input H3-BP was converted to water-soluble products and the remainder to diol-like metabolites. During the same time interval, WI-38 cells did not metabolize any significant amount of 7,12-dimethylbenz (a) anthracene. In collaboration with Dr. Roberto Revoltella and Dr. Luisa Bertolini, a new method for detecting cell surface membrane antigens has been developed. In this assay, lymphocyte blast transformation induced by fixed target cells is measured quantitatively, and histocompatibility differences between allogeneic cells and tumor-specific antigens on syngeneic cells can be recognized.